GIACIATION OF LOWER COPPER RIVER 463
Unfortunately the nature of the buried vegetation in the borings at Flag Point is not specified further, but, whether the vegetation was peat from a salt marsh or freshwater marsh, or stranded tree trunks, it is clear proof of sinking of the land. It is inconceivable that water-lodged trees, for example, should happen to be buried in one layer that is now 30 or 40 feet below sea level in all four of the sections and that elsewhere in the deposits there should be none.
This proof that the last movement of this coast line was downward is of much interest because there has also been post-glacial uplift of this same coast, shown on Wingham Island forty miles to the southeast, where there are uplifted silts containing striated, glacial pebbles and marine fossils.1
Extent of Outwash Deposits. The deposits of outwash laid down by glacial streams on the Copper River delta itself, on the valley train of Martin River Glacier, and on the outwash plain of Sheridan and Scott Glaciers form an area of approximately 500 square miles. They vary in character from bowldery conglomerates and coarse gravels at the head of the delta near Childs and Miles Glaciers to fine sand, silt, mud, and clay in the outer Copper River delta. They vary also in shape, the upper portion of the Copper and Martin River deposits forming a smooth outwash plain, the lower deposits of the same rivers forming a smooth delta, while the outwash gravels of the Sheridan, Scott and adjacent glaciers form an outwash apron of coalescing alluvial fans.
Valley Train of Martin River. The outwash deposits of the Martin River Glacier west of the terminus of that glacier, have a length of 10 miles and a width of from 3$ to 5 miles up to the point where they join the outwash deposits of Copper River valley. They are densely forested, where stream deposition is no longer going on, but barren along the present streams. The course of the Martin River, which, in 1905, emerged from the glacier near the northern side, joined by braided streams from a small cascading glacier, curves southward across the whole valley, because of the presence of a recessional moraine three miles west of the present terminus of the glacier, which interrupts the even slope of the outwash gravels. The present position of Martin River, close to the southern side of its valley, has resulted in the building up of the valley train higher than the mouths of five or more tributary valleys, in each of which the water is ponded back, forming good-sized lakes which have been described by G. C. Martin.2
Alluvial Slope of the Western Edge of the Delia. The outwash deposits of Sheridan and Scott Glaciers form a confluent outwash plain with so even a slope that the Copper River and Northwestern Railway has been built across it with a perfectly straight stretch of track for over eleven miles. The surface rises and falls, however, in a series of broad, low, alluvial fans, but the track is built over the undulating surface without much grading. Along the line of the railway the highest point is 49 feet above sea level, and the lowest point 15 feet, so that the maximum relief is 34 feet. These fans are highest near the present streams, or the places where large streams formerly flowed for a long time, the differences in level between the axis of an individual alluvial fan and the depression on one side being 34 feet in 2 miles, 11 feet in 1 mile, etc., according to the line of the section with reference to the head of the fan.
There are five main alluvial fans on the outwash gravel plain of the Sheridan and
i Martin, G. C., Bull. 385, U. S. Ged. Survey, 1908, pp. 46, 64. • Bull. 335, U. S. Ged. Survey, 1908, p. 56.